Catalytic dewaxing of various refined petroleum product fractions is now well established. Initial proposals were set out in U.S. Pat. No. 3,668,113 and Oil and Gas Journal, Jan. 6, 1975, pages 69-73. Since then, catalytic dewaxing of middle distillate and lube fractions has become well known. See, for example, U.S. Pat. Nos. Re 28398, 4,181,598, 4,247,388 and 4,443,327. The MLDW (Mobil Lube Dewaxing) catalytic dewaxing process for producing low pour point lubricants by dewaxing a lube boiling range feed in the presence of hydrogen over an intermediate pore size zeolite as the dewaxing catalyst has now become established. See, for example, 1986 Hydrocarbon Processing Handbook, Hydrocarbon Processing, September 1986, page 90. The MLDW process is described in Oil and Gas Journal, May 26, 1980, 75-84 and by Chen et al in "Industrial Application of Shape-Selective Catalysis", Catal. Rev. - Sci. Eng. 28 (243), 185-264 (1986), especially 244-247, to reference is made for a description of the process.
As described in the Chen article, the MLDW process utilizes a two-reactor system in which the effluent from the dewaxing reactor is cascaded to a hydrotreating reactor in which the dewaxed lube product is stabilised by saturation of lube boiling range olefins produced during the dewaxing step and removal of heteroatoms containing impurities and color bodies. Saturation of residual aromatics may also take place at higher hydrotreating pressures, typically about 2000 psig (about 13890 kPa abs). The use of a hydrotreating step in combination with the dewaxing step is also described in U.S. Pat. Nos. 3,894,938 and 4,181,598 (Gillespie) to which reference is made for details of such processes.
During the dewaxing process, the dewaxing catalyst becomes deactivated, mainly by the deposition of coke (a highly carbonaceous hydrocarbon principally constituted by polycyclic aromatic compounds) and the accumulation of various catalyst poisons, principally heteroatom compounds, especially nitrogenous materials. This results in progressive loss of catalytic activity and selectivity for which compensation is made during the dewaxing cycle by progressively increasing the temperature. It is not, however, possible to continue the increase in temperature indefinitely because at higher dewaxing temperatures the stability of the products deteriorates. In practice, end-of-cycle (EOC) temperatures of about 650.degree.-675.degree. F. (about 343.degree. to 357.degree. C.) have become typical. After the dewaxing cycle has been completed, the dewaxing catalyst is given a restorative treatment to bring back its activity and selectivity. Oxidative regeneration is effective for this purpose and restores the catalyst by oxidative removal of coke and other catalyst inhibitors at relatively high temperatures.
Oxidative regeneration techniques are widely known and are described, for example, in U.S. Pat. Nos. 3,069,362, 3,069,363 and British Patent No. 1,148,545.
Another restorative technique is hydrogen reactivation, which is commonly employed between oxidative regenerations to remove accumulated coke or adsorbed material which can lower catalyst activity. Under the conditions employed in treatments of this kind, the hydrogen reacts with the coke to form hydrogen-enriched compounds which are more mobile and which are removed from the catalyst while adsorbed catalyst poisons are removed by the stripping action of the hydrogen. The hydrogen may be used as such or mixed with inert gases or gas mixtures such as nitrogen, methane, carbon dioxide, carbon monoxide or flue gas, as described, for example, in U.S. Pat. Nos. 4,358,395 and 4,508,836. The hydrogen reactivation treatment may be combined with other treatments, for example, with an alkylamine stripping step to remove nitrogenous poisons as described in U.S. Pat. No. 4,560,670.
The catalysts which have been treated by these hydrogen reactivation procedures used to remove accumulated coke are, naturally, those catalysts employed in processes in which coke is laid down. Thus, the hydrogen reactivation has been used on hydrocracking catalysts and dewaxing catalysts, as shown in U.S. Pat. Nos. 4,247,388 and 4,508,836. These treatments have not, however, been employed with catalysts used in hydrotreating or hydrofinishing processes in which coke deposition takes place slowly and, thus, frequent catalyst regeneration is not required. In processes of this kind, the catalyst comprises a porous substrate which is essentially non-acidic in character and which functions as a support for a metallic hydrogenation component, usually a metal of Groups VIA or VIIIA of the Periodic Table (IUPAC Table), for example, nickel, cobalt, molybdenum, tungsten, chromium, platinum or palladium. These metals are active catalysts for the hydrogenation reactions which are desired and which require no acidic functionality. Because little coke is produced during the hydrogenative processing, neither oxidative regeneration nor hydrogen reactivation has been considered to have any advantage for hydrotreating catalysts, especially since it might affect the distribution and functioning of the metal component on the catalyst. Accordingly, neither has been used with catalysts of this kind.
One problem which has been encountered in the hydrogen reactivation of the dewaxing catalyst is that the hydrogenated coke components which are removed from the catalyst by the hydrogen treatment are not completely purged from the dewaxing reactor: they remain sorbed on the catalyst and are released at the start of the next dewaxing cycle, with the initial dewaxing product. Because the dewaxing catalyst is at peak activity and selectivity at the start-of-cycle (SOC), the conventional practice is to process premium lube products, especially turbine oils, at this time in order to secure the enhanced product stability associated with the low operating SOC temperatures, typically about 500.degree.-560.degree. F. (about 260.degree.-293.degree. C.), it is these premium products which become contaminated with the reactivation products. The reactivation products are generally high boiling fractions due to their highly aromatic nature and they cannot, therefore, be readily separated from the lube products and because they are aromatic in character, they degrade the properties of the lubes significantly, especially in viscosity index (V.I.), viscosity and oxidation stability. These contaminated lube products therefore is generally discarded (slopped), which represents a considerable waste, especially of turbine oils and other high quality products which cannot be effectively produced during later portions of the dewaxing cycle. There is therefore a need for improving the lube dewaxing/hydrotreating process in order to reduce or eliminate start-up slopping and to improve the quality of the dewaxed lube products, especially at the start of the dewaxing cycle.